The present invention relates to a method of calibration for an automatic machine tool, and more particularly for a lathe.
In the machine tool art, it is known to use a "touch trigger probe", of the kind described in U.S. Pat. No. 4,153,998, to trigger data processing means to produce an indication of x,y and z co-ordinates of a stylus of the probe in response to the stylus touching a surface. Such probes are moved relative to the workpiece in a similar manner to the cutting tools themselves (e.g. on tool-holding turret) and may be used both for checking the dimensions of a workpiece after machining by the machine tool and for controlling the machine tool during its machining of the workpiece.
The measurement indications of x,y and z co-ordinates are taken with respect to a measurement origin of the machine tool. Prior to use, the probe may be calibrated or "datumed" by causing it to trigger against one or more reference surfaces at fixed positions. This allows the calculation of "probe offsets" between the effective contact surfaces of the stylus and a nominal measurement position of the turret or other tool-holder in which the probe is mounted. These probe offsets are used to perform correction on subsequent measurements taken with the probe. Assuming the accuracy of the fixed reference position, such a correction takes account of the difference between the nominal measured position and the effective stylus contact surface.
We have found that a potential inaccuracy in probe datuming may occur in some machine tools if there is inaccuracy of positioning between the reference surface and the measurement origin. This may arise if there is deformation of the bed of the machine tool, causing a relative movement of the measurement origin. Such deformation may occur, for example, as a result of loading or thermal effects on the bed of the machine tool.
In the case of a machining center, such problems do not normally arise, at least in the x and y axes, for the following reason. The cutting tool spindle of a machining center is on the tool-holder, and hence its axis of rotation is local to the probe. The effects of machine bed deformation on the position of the measurement origin therefore have the same effect on both the measured position of the probe and on surfaces of the workpiece cut by the cutting tool. It follows that surfaces cut in the workpiece by the tool can be used as reference surfaces to calibrate the probe offset, since the effects of deformation then cancel each other out and do not give rise to a problem. Such techniques are described, for example, in U.S. Pat. No. 4,281,385 and British Patent Application No. GB 2,108,715 A.
In the case of lathes, it is known to calibrate the probe offset in the manner described above, using reference surfaces provided on the workpiece-holding chuck. See for example U.S. Pat. No. 4,382,215, and "Users Handbook--Programs for Probes, Component Inspection and Tool Datuming with Fanuc 6TB Control", Part 3, pages 25-35, published in 1984 by Renishaw Electrical Limited. However, we have found that this gives rise to the above-noted problem due to machine deformation. The problem cannot be removed simply by datuming the probe against surfaces machined in a workpiece by the use of the lathe, because the axis of rotation of the chuck is remote from the tool holder which holds the probe. Such datuming processes, we now realise, involve an assumption that the position of the axis of rotation of the chuck or workpiece can be accurately known relative to the measurement origin. In practice, this is not accurately known so if the machine is subject to deformation, and the inaccuracy is not cancelled out in the manner that it would be with a machining center, because the axis of rotation is not local to the probe.